Please use a range-based for loop.

you don't need this anymore?

As you were using const iterators before: Shouldn't this be qAsConst(doc->m_pagesVector) to avoid unnecessary detaching?

Why not simply numFields += pIt->formFields() ?

use unique_ptr

This convoluted for loop basically get the i-th element of a QLinkedList, which is not designed for index accessing (and that is why there is no at(int) method).
IMHO a better way is:

• get the list of form fields
• compare the wanted index with the count of the form fields: if it is out of boundaries, return KJSUndefined directly
• otherwise, create an iterator fromthe begin, and increment it by the wanted index to get to the wanted element

Are we talking about the std::unique_ptr?

I followed the template in the other classes, why should we use unique_ptr?

Then we get rid of the initialized bool?

And this does not compile

I still have to check for all the pages.

But my new implementation looks more like with what you described

Yep, also unique_ptr will clear the allocated memory, if you run valgrind, it will point a leak which is not good.

g_displayProto needs to be a std::unique_ptr for this to compile.